The International Atomic Energy Agency says Iran has made a significant step toward building a reactor that Western experts say could provide a second path to possibly producing material for a nuclear bomb.
In a quarterly report obtained by Western news agencies on Wednesday, the U.N. nuclear agency said Iran has delivered the reactor vessel to the heavy water plant near the western city of Arak. It said the component had not yet been installed.
Western powers fear the Arak facility could provide Iran with plutonium for nuclear weapons if the reactor's spent fuel is reprocessed. Iran insists its nuclear ambitions are peaceful.
Western concerns about Iran's nuclear activities have focussed mostly on two facilities that enrich uranium, a process that has peaceful and military uses.
Iran's underground Fordo plant has been enriching uranium to purities of 20 percent, a relatively small step away from the higher purity needed for a nuclear bomb.
The IAEA report said enrichment work has stagnated at Fordo in the past three months.
But the report said Iran has accelerated enrichment at Natanz, with almost 700 advanced centrifuges installed in the facility by this month, compared to 180 in February.
The advanced equipment allows Iran to enrich uranium more quickly. But most uranium enrichment at Natanz is to the lower level of five percent purity, far short of weapons-grade.
Iran has defied four rounds of U.N. Security Council sanctions demanding a suspension of uranium enrichment.